Measurement of gas impurity concentration by infrared absorption spectroscopy

ABSTRACT

An impurity in a gas medium is detected and its concentration measured by an infrared absorption spectroscopy technique. An infrared laser beam, whose wavelength is at or near an absorption line of the suspected impurity, is amplitude modulated at a fundamental acoustic frequency; and the modulated beam is incident upon the gas medium. The amplitude of the second harmonic acoustic frequency component of the resulting induced pressure (or temperature) variation of the gas medium is indicative, with great sensitivity, of the presence and concentration of the impurity.

United States Patent Kruezer Oct. 24, 1972 [72] Inventor:

[541 MEASUREMENT OF GAS IMPURITY CONCENTRATION BY INFRARED ABSORPTION SPECTROSCOPY Lloyd Barton Kruezer, Bernardsville,N.J.

[73] Assignee: Bell Telephone Laboratories, Incorporated, Murray Hill, Berkeley Heights, NJ.

[22] Filed: Nov. 2, 1970 [21] App1.No.: 85,884

[52] US. Cl. ..250/43.5 R [51] Int. Cl. ..G0ln 21/26 [58] Field of Search ..'..250/43.5 R; 356/89 [56] References Cited UNITED STATES PATENTS 3,039,353 6/1962 Coates et al. ..356/89 3,287,556 11/1966 Good ..250/43.5 R 3,171,027 2/1965 Wallack ..250/43.5 R

MODULATOR 2,938,118 5/1960 Martin ..250/43.5 R 3,215,832 11/1965 Madsen et a1 ..250/43.5 R 3,447,876 6/ 1969 Barringer ..250/43.5 R 3,162,761 12/1964 Luft ..250/43.5 R

Primary Examiner-James W. Lawrence Assistant Examiner-Davis L. Willis Attorney-R. J. Guenther and Arthur J. Torsiglieri [57] ABSI'RACT An impurity in a gas medium is detected and its concentration measured by an infrared absorption spectroscopy technique. An infrared laser beam, whose wavelength is at or near an absorption line of the suspected impurity, is amplitude modulated at a fundamental acoustic frequency; and the modulated beam is incident upon the gas medium. The amplitude of the second harmonic acoustic frequency component of the resulting induced pressure (or temperature) variation of the gas medium is indicative, with great sensitivity, of the presence and concentration of the impurity.

9 Claims, 1 Drawing Figure NVENTO/P L. B. KREUZER B) ATTORNEY PAIENTED 0m 24 I972 MEASUREMENT OF GAS IMPURITY CONCENTRATION BY INFRARED ABSORPTION SPECTROSCOPY FIELD OF THE INVENTION This invention relates to the field of optical absorption spectroscopy, and more particular to infrared absorption systems for detecting impurities in a gaseous medium.

BACKGROUND OF THE INVENTION Many years ago, A. G. Bell, Proc. Am. Assoc. Adv. Science, Vol. 29, page 115 (1880), Phil Mag. Vol 11, page 510 (1881); J. Tyndall, Proc. Roy. Soc. Vol. 31, page 307 (1881); and W. C. Rontgen, Phil. Mag. Vol. 11, page 308 (1881 discovered the opto-acoustic effeet and its use in the spectrophone. Briefly, input optical radiation, periodically interrupted at a frequency in the audible range, was directed upon a gas medium in a glass container; and the periodic pressure fluctuations resulting from the absorption of radiation by the gas was detected by ear through a listening tube connected to the container. This effect is ordinarily most pronounced when a substantial portion of the input radiation is in the infrared portion of the electromagnetic. spectrum, where the radiative lifetime of the gas molecules is larger than the relaxation time for the energy of excited molecules to be converted into heat, so that a substantial portion of the (periodic) input radiation is converted into (periodically) heating the gas. Since the infrared absorption spectra of different molecules in a gas depend upon the nature of the vibration and rotation states of the various molecules, the magnitude of the acousto-optical effect is a function of the input infrared wavelength as well as the composition of the gas and can be used to analyze this composition.

Using thermal sources of infrared radiation in combination with wavelength selective filters and mechanical choppers, the opto-acoustic effect is presently used in commercial systems for the purpose of analyzing for impurities in gases, such as the presence of carbon dioxide in air. In such systems,'for example, in infrared beam is provided having infrared wavelength components equal to the wavelengths of infrared absorptions of the impurity (carbon dioxide). The amplitude of the beam is modulated at an acoustic frequency to form a modulated input infrared beam. This input beam is incident upon the sample of air to be tested for the presence of .the impurity. An output acoustic frequency, equal to the modulating frequency, is detected with a difference microphone. This microphone is responsive to the difference in pressure between the test sample of gas and a pure sample of gas upon which the input beam is also incident. The sensitivity of such techniques in such systems is limited by the fact that the signal to noise ratio in the acoustic output at the modulating frequency of the infrared beam (that-is, for example the ratib of the absorption by the carbon dioxide relative to the air) is limited by the contribution to the absorption process (and hence, to the acoustic output) by the major constituents in the gas, even if the wavelength of the input infrared beam is monochromatically centered at an absorption line of the impurity to be detected. With the increasing need and importance of detecting microscopic traces of impurities in the air, for monitoring and controlling the pollution of our environment, it is desirable to have a more sensitive system for detecting impurities in gases.

Accordingly, it is an object of this invention to provide apparatus for detecting gaseous impurities, with greater sensitivity than in the prior art.

SUMMARY OF THE INVENTION This invention is based upon the realization and utilization of a phenomenon associated with the acoustic output arising from modulation of an infrared beam. 'In particular, if the input infrared beam has an intensity sufficiently strong to cause nonlinearity (preferably quadratic) in the absorption process in the impurity, the acoustic output signal from the impurity at a harmonic frequency (preferably the second) of the modulation frequency increases nonlinearly (preferably quadratic), whereas the acoustic background due to the other constituents increases only linearly with the intensity of the input infrared beam. To this end, the invention involves use of an input infrared source of sufficient intensity to cause such nonlinearity in the absorption process, together with detection of the acoustic output of a harmonic (preferably the second) of the modulation frequency.

In a specific embodiment of this invention, an intense infrared beam (A 3.39 micron) is chopped at a fundamental frequency of the order of Hz. This chopped beam is incident upon an absorption cell filled with a sample of air to be tested for methane. The cell is in the shape of a cylinder whose walls are made of an apertured layer of structural metal. This layer of metal is surrounded by a dielectric layer of teflon, upon which is an evaporated layer of aluminum. Fluctuations in the pressure of the air at the second harmonic of the fundamental are produced in proportion to the amount of methane in the gas. These fluctuations in turn modulate the electrical capacitance of the capacitor formed by the structural metal dielectric, and aluminum layer. An electrical amplifier and electrical detector of signals at the second harmonic respond to these changes in capacitance and thereby .reveal the presence and amount of methane in the air.

BRIEF DESCRIPTION OF DRAWING This invention can be better understood from the following detailed description when read in conjunction with the drawing in which the FIGURE shows, partly in cross section, a system for detecting impurities in a gas, in accordance with a specific embodiment of the invention. For the sake of clarity only, the FIGURE is not to scale.

DETAILED DESCRIPTION As shown in the FIGURE, an absorption cell 20 is filled with air or other gas, typically at a pressure of about 100 mm Hg or more, containing methane as an impurity to be detected. An intense infrared heliumneon laser source 11 provides a beam 11.1 having a wavelength of 3.39 micron which is an absorption infrared line of methane. A modulator l2, typically a Pockel cell or acousto-optic device, modulates the intensity of the beam 11.1 at a fundamental acoustic frequency f,, preferably in the range of about 20 Hz to 500 Hz, for example, 100 Hz. The modulation is preferably sinusoidal, and in all eventsit is important that substantially no second harmonic frequency f: (=2 f1) of the fundamental be present in theFouriercomponents of the modulation. An amplitude modulated infrared beam 11.2, exits fromthe chopper 12 and traverses the absorption cell 20, exiting therefrom as beam 1 1.3 as indicated in the FIGURE.

The absorption cell includes v a cylindrically shaped metal tube 22 containing the gasto be tested. The tube 22 is provided with. a plurality of apertures for communication of the gas with a cylindrically-shaped dielectric insulator layer 23 surrounding the metal tube 22. Typically, the insulator layer 23. is essentially teflon, but electret materials can be used to support even higher voltage biases for greater sensitivity of micr'ophonedetection. A cylindrically shaped metal layer'24, typically evaporated aluminum, surrounds the insulator layer 23. The microscopic distance between the layers 23 and 24 from the walls of the tube 22 is exaggerated in the FIGURE, for purposes of showing that some of the gas within the tube 22 can leak out into the microscopically small space therebetween, in response to pressure variations in this gas, thereby forming a variable capacitance microphone. The tube 22 is sealed at both ends by glass windows 21.1 and 21.2, whichare transparent to the infrared beam 11.2. A battery 25 supplies a voltage bias to the tube 22 and the metal layer 24. Electrical signals are produced by the microphone caused by varying capacitance of the capacitor which is formed by the walls of the cell 22, the dielectric layer 23 together with the variable amount and pressure of the gas in the space therebetween, and the metal layer 24. These electrical signals in turn are excited by the pressure variation in the gas in response to the modulated laser beam 11.2. Amplification of these electrical signals-is provided by an amplifier 28, advantageously including a feedback capacitor 27 of capacitance C, and a feedback resistor 26 of resistance R After amplification, these electrical signals are subjected to an electrical filter 29 which is of the bandpass type preferably centered at the second harmonic of (Le, twice) the fundamental frequency produced in the laser beam 11.2 by the modulator 12. The output of the filter 29 is fed to an AC. detector 30 of the electrical output of the filter 29 (centered at the passband frequency). Typically, the detector 30 is a synchronous detector.

Optimization of the output acoustic second harmonic is obtained when the intensity of the infrared beam is approximately equal to one-half the beam intensity which would maximize the output acoustic fundamental frequency.

When the modulation of the infrared input beam is sinusoidal, the intensity I satisfies:

molecule is either in the upper or lower state of the transition. Then, the optimum 10 in photons per unit area satisfies:

proximates the diameter of the laser beam 11.2-

propagating therethrough, so that the beam and the tube are in registry. In this way, the signal from the variable capacitance rnicrophone is optimized. Typically, the diameter D of the tube 22 is of the order of l mm, which is a convenient-size for the cross section of the laser beam 11.2. Since the microphone signal to noise ratio improves as (LD)" where L is the length of the tube 22, L is preferably as large as possible, consistent with mechanical and optical alignment. Typically, therefore, L is of the order of l to 5 cm.

Turning to the advantageous values of R, and C, in the amplifier 28, the voltage gain is proportional to C /C, where C is the capacitance of the microphone, typically of the order of 200 p.f. Thus, C, should be smaller than C,,,. On the other hand, R; should be small enough 'to prevent static charge accumulation from saturating the amplifier 28. Taking all these factors into consideration, with an acoustic fundamental frequency f 400 Hz, typically of the capacitance of C, is selected to be of the order of l to 10 p.f. while'the resistance of R; is selected to be of the order of 10 ohms.

Although this invention has been described in detail in terms of a particular embodiment, various modifications can be made by the worker of ordinary skill in the art without departing from the scope of the invention. For example, any sufficiently intense source of optical radiation can be used instead of the laser 11, such as tunable optical parametric devices. Likewise, other gaseous impurities, such as C0, C0 N 0, in air, can be detected in the cell 20 using optical sources 11 containing wavelength(s) corresponding to the infrared abponent of a frequency which is a harmonic relative t0 the frequency of the fundamental frequency at which the beam is being modulated; and

. means for detecting the said harmonic component response of the medium to the beam.

2. The combination recited in claim 1 in which the beam of electromagnetic radiation is characterized by a maximum of intensity at an infrared frequency component.

3. The combination recited in claim 1 in which the harmonic frequency is equal to twice the fundamental frequency.

4. The combination recited in claim 3 in which the infrared frequency component is supplied by an optical parametric device.

5. The combination recited in claim 3 in which the fundamental frequency is in the acoustic range.

6. The combination recited in claim 5 in which the means for detecting includes a microphone.

7. The combination recited in claim 3 in which the infrared frequency is at least approximately equal to the frequency of an absorption line of the gaseous impurity.

8. A method for detecting an impurity in a medium which comprises the steps of:

a. directing upon the medium a beam of electromagnetic radiation which includes a wavelength corresponding to an absorption line of the impurity and which is modulated in intensity at a fundamental frequency whereby the beam induces a time varying pressure response in the medium, said pressure response containing at least one harmonic component relative to the frequency of the fundamental frequency at which the beam is being modulated; and

b. monitoring the said harmonic component response of the medium.

9. The method recited in claim 8 in which the beam comprises an infrared beam having a maximum of intensity at a frequency at least approximately equal to the frequency of an absorption line of the impurity, and the harmonic is equal to twice the fundamental frequency. 

1. In combination: a. an absorption cell containing a gas medium to be analyzed for a gaseous impurity; b. means forming a beam of electromagnetic radiation containing a wavelength corresponding to an absorption line of the impurity, the intensity of said beam being modulated at a fundamental frequency whereby the beam induces a time varying pressure response in the medium, said pressure response containing at least one harmonic component of a frequency which is a harmonic relative to the frequency of the fundamental frequency at which the beam is being modulated; and c. means for detecting the said harmonic component response of the medium to the beam.
 2. The combination recited in claim 1 in which the beam of electromagnetic radiation is characterized by a maximum of intensity at an infrared frequency component.
 3. The combination recited in claim 1 in which the harmonic frequency is equal to twice the fundamental frequency.
 4. The combination recited in claim 3 in which the infrared frequency component is supplied by an optical parametric device.
 5. The combination recited in claim 3 in which the fundamental frequency is in the acoustic range.
 6. The combination recited in claim 5 in which the means for detecting includes a microphone.
 7. The combination recited in claim 3 in which the infrared frequency is at least approximately equal to the frequency of an absorption line of the gaseous impurity.
 8. A method for detecting an impurity in a medium which comprises the steps of: a. directing upon the medium a beam of electromagnetic radiation which includes a wavelength corresponding to an absorption line of the impurity and which is modulated in intensity at a fundamental frequency whereby the beam induces a time varying pressure response in the medium, said pressure response containing at least one harmonic component relative to the frequency of the fundamental frequency at which the beam is being modulated; and b. monitoring the said harmonic component response of the medium.
 9. The method recited in claim 8 in which the beaM comprises an infrared beam having a maximum of intensity at a frequency at least approximately equal to the frequency of an absorption line of the impurity, and the harmonic is equal to twice the fundamental frequency. 